1. Field of Invention
The present invention relates to femoral hip prostheses, and more particularly to a femoral hip stem component having a shape which provides a better fit within the femoral medullary canal.
2. Background of Invention
Hip arthroplasty procedures involve the implantation of a prosthetic stem component within a femoral intramedullary canal. A ball on the proximal end of the stem cooperates with a socket of an acetabulum to provide for articulation between the femur and the acetabulum. In order to maintain pain-free articulation of the hip joint following implantation of the stem, it is important that the stem be securely fastened to the intramedullary canal. Such fastening can be accomplished with a bone cement which adheres to the stem and the wall of the intramedullary canal. In addition numerous stems have been provided with a porous surface as taught in U.S. Pat. No. 4,550,448 (Kenna) and U.S. Pat. No. 3,605,123 (Hahn) to either accommodate adherence with the bone cement or enhance a press fit between the porous surface and the wall of the intramedullary canal. If a press fit is desired with the intramedullary canal, the stem contour should closely match the contour of the intramedullary canal so that the porous surface is in intimate contact with bone, thereby enabling bone to grow into the porous surface.
Various patents relate to a femoral component for press fit with, and biological fixation to, the wall of the proximal metaphysis and intramedullary canal. U.S. Pat. No. 4,589,883 (Kenna) and U.S. Pat. No. 4,738,681 (Koeneman et al.) teach a femoral stem having a twist in the proximal region for improved fit and stability within the femoral canal. While this appears to be close to the geometry of the natural femur, in practice the rotational motion of the stem induced by the twist can lead to enlargement of the implantation site and the formation of gaps at the implant/bone interface. The twist in this region also prevents the stem from sitting within the neck of the femur. Instead, the stem sits in the bone in a rotated position, thus making preparation of the implantation site more difficult since surgeons often attempt to change the rotational position of the implant to restore the normal position of the femoral head.
A second limitation existing in the art is the cross-sectional shape of the stem, in particular in the proximal regions of the stem, wherein the geometry often necessitates the removal of strong bone within the femur (e.g. the calcar femorale and the medial border of the greater trochanter) before the stem can be correctly implanted, achieving a close fit to the canal. In practice, this is difficult to achieve with existing surgical instruments, so many such prostheses are difficult to implant without undersizing or misalignment. U.S. Pat. No. 5,358,534 (Dudasek, et al.), as well as Kenna described above, teach a stem wherein a transverse cross-section taken in the proximal region of the stem is substantially rectangular in shape (i.e. it has parallel anterior and posterior edges). Such a shape does not conform well to the internal anatomy of the femoral intramedullary canal and necessitates the removal of the bone from the greater trochanter during implantation. In addition, while the Dudasek stem does disclose the presence of a posterior concavity to clear the posterior cortex of the intramedullary canal, it does not teach or suggest the geometry or dimensions of the concavity for allowing the medullary cavity to be maximally filled with the stem without the need to remove the calcar femorale.
U.S. Pat. No. 4,813,963 (Hori, et al.) is directed to a stem having a configuration that, according to the specification, more accurately reflects the anatomic contour of the intramedullary canal. In particular, the patent teaches a stem wherein the proximal portion, in transverse cross-section, has an asymmetric contour to define an anterior side which forms an acute angle with a lateral side, and a posterior side which approaches the anterior side in the medial direction. In addition, the medial side is arcuate in shape while the other sides have linear edges. However, this stem still requires removal of bone from the greater trochanter due to the wide angle of the anterior/lateral edge and the bulk in the posterior/lateral corner. Moreover, although designers of previous prosthetic devices have utilized simple cross-sectional shapes with flat sides to facilitate manufacture of these implants, the inner contours of the femur are arcuate and rarely linear. Consequently, stems such as that taught by Hori, et al. only achieve contact with the femoral cavity at discrete points, typically along relatively sharp edges. This may result in localized stem concentration and could lead to an increased incidence of bone fracture during implantation. In addition, areas of relatively soft bone will be left between the localized points of contact. These areas tend to become osteoporotic with time, leading to less biological attachment of the prosthesis through bony ingrowth. These areas may also act as open channels within the bone structure, making the femur susceptible to infiltrating particles generated by wearing of the artificial joint. The biological reaction to these particles quite frequently leads to erosion of bone around the prosthesis and may cause loosening and failure of the implant.
A third problem with the prior art is thigh pain that is often experienced after cementless total hip arthroplasty. This pain is commonly linked to the stiffness mismatch between the bone and the prosthesis. Provision of a distal slot reduces bending stiffness, thereby reducing subsequent thigh pain. Other beneficial effects of reducing distal bending stiffness include easier stem insertion, lower incidence of distal fractures during stem insertion, and greater ease of implanting the correct size implant. Conventional slotted prostheses, such as those of Thongpreda et al. (EPO 0 543 099 A2) and Noiles (U.S. Pat. No. 3,996,625) have slots oriented in the coronal plane, and therefore only provide the foregoing benefits in one plane of bending. In most activities, the force acting on the distal stem is not directed anteriorly or posteriorly, but has a significant medial-lateral component.